The insulin trap: A and B chains are NOT made in one cell.
When NEET asks about recombinant insulin (Humulin), the most common mistake is assuming both polypeptide chains are produced together inside a single E. coli cell. They are not.
What NCERT says (Class 12 Biology, Chapter 10, page 181):
Human insulin consists of two polypeptide chains — chain A (21 amino acids) and chain B (30 amino acids). These are linked by disulfide bridges. In the human pancreas, insulin is initially synthesised as proinsulin — a single polypeptide that includes chain A, chain B, and a connecting C-peptide. The C-peptide is removed during maturation.
The biotechnology approach:
Eli Lilly produced Humulin in 1983 using a key strategy: the genes for chain A and chain B were introduced into two separate E. coli strains. Each strain expressed only one chain. The chains were then extracted, purified, and combined in vitro, where disulfide bonds formed between them to produce functional insulin.
Why not express proinsulin directly? Early E. coli systems could not reliably process the C-peptide removal. Separate expression avoided this folding problem entirely.
What NEET targets:
Questions test whether you know: (a) insulin has two chains linked by disulfide bonds, (b) recombinant production uses separate bacterial cultures for each chain, (c) assembly happens outside the cell, and (d) the distinction between proinsulin (with C-peptide) and mature insulin (without it).
Molecular diagnosis — PCR and ELISA: Beyond recombinant insulin, NCERT groups PCR and ELISA together as molecular diagnosis, catching a disease before the pathogen's concentration in the body is high enough for symptoms to appear. PCR amplifies a pathogen's nucleic acid, so even a very low amount of bacterial or viral DNA is detectable; it is now routinely used to detect HIV in suspected AIDS patients and mutations in suspected cancer patients. ELISA works on a different principle — antigen-antibody interaction — and detects infection either through the pathogen's own antigens or through the antibodies the body makes against it (NCERT Class 12 Biology, Chapter 10, page 183).
Watch out: A distractor claiming "A and B chains are co-expressed and self-assemble inside E. coli" exploits the gap between knowing the chain structure and knowing the production method. The assembly is extracellular — that is the fact NEET rewards.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Human insulin consists of how many polypeptide chains, and what connects them?
Answer: D. Mature human insulin has two polypeptide chains — A (21 amino acids) and B (30 amino acids) — held together by disulfide bridges (NCERT Class 12 Biology Chapter 10, pages 181–182).
Why A is wrong: A is wrong because insulin is not a single-chain molecule; it has two distinct polypeptide chains (A and B) in its mature form.
Why B is wrong: B is wrong because phosphodiester bonds link nucleotides in DNA/RNA. Insulin's chains are connected by disulfide bonds between cysteine residues.
Why C is wrong: C is wrong because the C-peptide is present in proinsulin but is removed during maturation. Mature insulin has only two chains, not three.
What is the C-peptide in the context of insulin biosynthesis?
Answer: C. The C-peptide is the connecting segment between chain A and chain B in proinsulin. It is excised during processing to yield mature, functional insulin (NCERT Class 12 Biology Chapter 10, page 182).
Why A is wrong: A is wrong because the C-peptide is part of the proinsulin polypeptide, not a separate gene for the insulin receptor.
Why B is wrong: B is wrong because the C-peptide is a peptide segment, not a carbohydrate. Insulin does not undergo glycosylation.
Why D is wrong: D is wrong because the signal peptide (if present) is a different N-terminal sequence; the C-peptide specifically connects chain A and chain B within proinsulin.
Which company first produced recombinant human insulin (Humulin) commercially?
Answer: D. Eli Lilly produced Humulin in 1983, the first commercially available recombinant human insulin (NCERT Class 12 Biology Chapter 10, pages 181–182).
Why A is wrong: A is wrong because Novo Nordisk, while a major insulin producer, was not the first to produce recombinant human insulin commercially.
Why B is wrong: B is wrong because Amgen is a biotechnology company focused on other therapeutic proteins (e.g., erythropoietin), not recombinant insulin.
Why C is wrong: C is wrong because Genentech contributed to the research on cloning the insulin gene but did not commercially produce Humulin.
In Eli Lilly's production of Humulin, the A and B chains of insulin were:
Answer: A. Eli Lilly's method involved introducing the gene for chain A into one E. coli strain and the gene for chain B into another. The separately produced chains were then extracted and combined outside the cell, with disulfide bonds forming in vitro (NCERT Class 12 Biology Chapter 10, page 182). This directly tests the common mistake of assuming co-expression in a single cell.
Why B is wrong: B is wrong because the chains were NOT co-expressed in a single cell. This is the common mistake — believing that both chains self-assemble inside one bacterium. The assembly is extracellular.
Why C is wrong: C is wrong because Eli Lilly's original Humulin process used E. coli, not yeast, and expressed the chains separately rather than as proinsulin with C-peptide.
Why D is wrong: D is wrong because Humulin was produced using recombinant DNA technology in living E. coli cells, not by chemical synthesis.
Why were the A and B chains of recombinant insulin expressed in separate E. coli cultures rather than as a single proinsulin polypeptide?
Answer: B. Early E. coli systems lacked the post-translational processing needed to cleave the C-peptide from proinsulin. Expressing the chains separately bypassed this limitation; chains were then combined and disulfide bonds formed in vitro (NCERT Class 12 Biology Chapter 10, page 182).
Why A is wrong: A is wrong because E. coli CAN transcribe eukaryotic genes when they are provided as cDNA (intron-free) under a bacterial promoter. The issue was post-translational processing, not transcription.
Why C is wrong: C is wrong because the insulin coding sequence is small (a few hundred base pairs) and easily fits in standard cloning vectors. Size was not the limitation.
Why D is wrong: D is wrong because while the E. coli cytoplasm is reducing (making disulfide bond formation difficult), the statement that disulfide bonds 'cannot form under any conditions' is too absolute. The actual reason for separate expression was the C-peptide processing problem.
A student claims: "Proinsulin and mature insulin are identical molecules." Which correction is accurate?
Answer: B. Proinsulin is a single polypeptide containing chain A, chain B, and the C-peptide. During maturation, the C-peptide is removed, leaving the two-chain mature insulin held by disulfide bridges (NCERT Class 12 Biology Chapter 10, page 182).
Why A is wrong: A is wrong because proinsulin also contains disulfide bonds — they form before C-peptide removal. The difference is the presence of the C-peptide, not the disulfide bonds.
Why C is wrong: C is wrong because neither proinsulin nor mature insulin is synthesised in the nucleus. Proinsulin is translated on ribosomes (rough ER) and processed in the Golgi.
Why D is wrong: D is wrong because proinsulin is a single polypeptide (not three chains), and mature insulin has two chains (A and B), not one.
If a biotechnology company wanted to produce functional human insulin by expressing proinsulin as a single polypeptide in E. coli, what additional processing step would be essential before the product could be used therapeutically?
Answer: A. Proinsulin expressed in E. coli retains the C-peptide. To produce mature, active insulin, the C-peptide must be enzymatically excised and the correct disulfide bonds between chains A and B must form. This is the processing step that early E. coli systems could not perform endogenously, which is why Eli Lilly chose the two-strain approach (NCERT Class 12 Biology Chapter 10, page 182).
Why B is wrong: B is wrong because E. coli does not process introns (no spliceosome). The gene inserted is cDNA (intron-free). Adding introns to mRNA is not a relevant processing step.
Why C is wrong: C is wrong because insulin is not a glycoprotein. Glycosylation is not part of insulin maturation.
Why D is wrong: D is wrong because phosphorylation of the B chain is not a step in insulin processing. The key post-translational event is C-peptide removal and disulfide bond formation.
Consider the following statements about recombinant insulin production:
(i) Chain A has 21 amino acids; chain B has 30 amino acids.
(ii) The two chains are expressed in the same E. coli cell and assembled intracellularly.
(iii) Mature insulin is formed by removing the C-peptide from proinsulin.
(iv) Disulfide bonds link the A and B chains in the functional hormone.
Which statements are correct?
Answer: C. Statement (i) is correct — chain A has 21 and chain B has 30 amino acids. Statement (iii) is correct — C-peptide removal converts proinsulin to mature insulin. Statement (iv) is correct — disulfide bridges connect the two chains. Statement (ii) is wrong — in Eli Lilly's process, the chains are expressed in separate E. coli strains and combined in vitro, not assembled intracellularly in one cell (NCERT Class 12 Biology Chapter 10, page 182).
Why A is wrong: A is wrong because it includes statement (ii). The A and B chains are NOT expressed in the same E. coli cell — they are produced in separate strains and combined extracellularly.
Why B is wrong: B is wrong because it includes statement (ii). Even though (iii) and (iv) are correct, statement (ii) is false — the chains are produced separately, not in the same cell.
Why D is wrong: D is wrong because it includes statement (ii). Statement (ii) contradicts the established Eli Lilly production method where separate bacterial cultures are used for each chain.
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Given
A question states: "In the production of recombinant human insulin, Eli Lilly expressed the A and B polypeptide chains of insulin in two separate E. coli strains. Explain why this approach was used instead of expressing proinsulin directly, and describe how functional insulin is obtained from the separately produced chains."
Required
Explain the rationale for separate-chain expression and describe the in vitro assembly process.
Concept
Human insulin is initially produced in the pancreas as proinsulin — a single polypeptide containing chain A (21 amino acids), chain B (30 amino acids), and a connecting C-peptide. Mature insulin is formed when the C-peptide is enzymatically removed and disulfide bonds link the two chains. Early E. coli expression systems lacked the post-translational machinery to cleave the C-peptide correctly.
Formula/principle
No mathematical formula applies. The governing principle is: E. coli can transcribe and translate eukaryotic cDNA but cannot perform complex post-translational processing (C-peptide excision).
Substitution/application
Eli Lilly introduced the chain A gene into one E. coli strain and the chain B gene into a separate strain. Each culture produces its respective polypeptide independently.
Calculation/reasoning
After expression and extraction:
1. Chain A and chain B are purified separately.
2. The purified chains are mixed under controlled oxidising conditions in vitro.
3. Disulfide bonds form between the correct cysteine residues, producing the two-chain insulin molecule.
4. The product (Humulin) is structurally identical to native human insulin.
Final answer
The two-strain approach was necessary because early E. coli could not process proinsulin's C-peptide. By expressing chains separately and assembling them extracellularly via disulfide bond formation, Eli Lilly produced functional human insulin identical to the native hormone.
Common trap
The common mistake is claiming that both chains are expressed together in one E. coli cell and self-assemble intracellularly. The assembly is extracellular (in vitro) — this is the fact NEET distractors exploit.
Similar NEET-style question
"Assertion (A): Recombinant insulin produced by E. coli is structurally identical to human insulin.
Reason (R): Both A and B chains are co-expressed in the same E. coli cell, allowing natural folding.
Choose the correct option."
Answer: A is true, R is false. The chains are expressed in separate E. coli strains and combined in vitro. The product is indeed identical to human insulin, but the reason stated is incorrect.
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Recombinant insulin: Eli Lilly 1983 (Humulin) — A + B chains in E. coli, joined by disulfide. Gene therapy: ADA-deficient SCID (1990, Ashanti DeSilva). ELISA: enzyme-linked immunosorbent assay (HIV diagnosis). Molecular diagnosis: PCR for early detection.
-- NCERT Class 12 Biology, Chapter 10, p. 182These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: concept gap
Eli Lilly's Humulin: A and B chains expressed SEPARATELY in two E. coli strains; chains then combined in vitro and joined by disulfide bonds.
More in Biotechnology and its Applications: 6 exam traps and mistakes · 1 question pattern from its other lessons.
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